1. Field of the Invention
This invention relates to the field of microelectromechanical (MEM) devices, and particularly to methods of fabricating MEM devices for use in high temperature environments.
2. Description of the Related Art
Microelectromechanical (MEM) devices—i.e., integrated devices which include at least one moveable element that moves relative to a stationary element—have become commonplace. Such devices have found many applications, including switches, tunable capacitors, current sensors, viscosity sensors, accelerometers and the like.
One approach to fabricating MEM devices, described, for example, in U.S. Pat. No. 6,159,385 and U.S. Patent Application Publication U.S. 2004/0113513, involves the use of a silicon-on-insulator (SOI) wafer and a substrate. The SOI wafer, which includes a layer of doped silicon and an oxide layer, is bonded to the substrate using an organic adhesive to form a composite structure. The doped layer is patterned and etched to form the moveable elements, and the structure is then processed to undercut the adhesive and release the moveable elements. A metallization layer is typically deposited, masked and etched to provide electrical interconnections for the MEM device.
However, devices fabricated in this manner may fail if subjected to high temperatures. For example, a MEM viscosity sensor, such as that described in co-pending patent application Ser. No. 10/956,229 filed on Sep. 30, 2004 and assigned to the same assignee as the present application, must be immersed in the fluid being monitored. If the fluid temperature is high, the device's organic adhesive can degrade and compromise the device's structural integrity. In addition, the metallization layer, typically aluminum-based, may also be adversely affected by exposure to high temperature fluids.